System and method for cryogenic optoelectronic data link

ABSTRACT

A cryogenic optoelectronic data link, comprising a sending module operating at a cryogenic temperature less than 100 K. An ultrasensitive electro-optic modulator, sensitive to input voltages of less than 10 mV, may include at least one optically active layer of graphene, which may be part of a microscale resonator, which in turn may be integrated with an optical waveguide or an optical fiber. The optoelectronic data link enables optical output of weak electrical signals from superconducting or other cryogenic electronic devices in either digital or analog form. The modulator may be integrated on the same chip as the cryogenic electrical devices. A plurality of cryogenic electrical devices may generate a plurality of electrical signals, each coupled to its own modulator. The plurality of modulators may be resonant at different frequencies, and coupled to a common optical output line to transmit a combined wavelength-division-multiplexed (WDM) optical signal.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 16/867,462, filed May 5, 2020, now U.S. Pat. No. 11,115,131,issued Sep. 7, 2021, which is a Continuation of U.S. patent applicationSer. No. 16/153,659, filed Oct. 5, 2018, now U.S. Pat. No. 10,644,809,issued May 5, 2020, which is a Continuation of U.S. patent applicationSer. No. 15/356,030, filed Nov. 18, 2018, now U.S. Pat. No. 10,097,281,issued Oct. 9, 2018, which is a non-provisional of, and claims benefitof priority from U.S. Provisional patent application Ser. No.62/256,991, filed Nov. 18, 2015, the entirety of which is expresslyincorporated herein by reference.

BACKGROUND OF THE INVENTION

There are several low-power cryogenic electronic technologies thatrequire broadband communication of analog or digital data toconventional electronic systems at room temperature. These includeultrafast superconducting circuits for data conversion, radiocommunications, and computing, as well as superconducting circuits forquantum computing and sensitive imaging arrays across theelectromagnetic spectrum. A key problem with these systems is that thecharacteristic voltage output of these low-power systems is 1 mV or evenless, while room-temperature semiconductor electronic devices have atypical voltage level of order 1 V or more. In some cases, one can use ahigh-gain transistor amplifier to bridge this gap, but such transistoramplifiers may dissipate too much power for the cryogenic environment.Furthermore, the electrically conductive high-bandwidth transmissionoutput lines themselves may conduct too much heat to the cryogenicenvironment. (See FIG. 1A.) See, for example, Wuensch et al., “CryogenicSemiconductor Amplifier for RSFQ Circuits with High Data Rates at 4K”,IEEE Transactions on Applied Superconductivity, vol. 19, pp. 574-579,2009; Gupta et al., “Low-power High-Speed Hybrid TemperatureHeterogeneous Technology Digital Data Links”, IEEE Transactions onApplied Superconductivity, vol. 23, article 1701806, 2013.

The key issues for such devices include how much power dissipation isrequired in or near the cryocooler which maintains the superconductiveelectronics at operating temperature, how much heat transfer occurs intothe cryocooler through the transmission lines, and how well thetransmission lines perform. Ancillary issues relate to how large and/orcomplex are, and what type of circuits within the cryocooler arerequired to drive the transmission lines.

A promising alternative to electrical transmission lines is opticalcommunication via optical fibers, which can be constructed to have highperformance while conducting very little heat. Note that these opticaltransmission lines need not be very long, and therefore the opticalsignal may be weak as compared to long distance room temperaturetelecommunication fibers. This requires an electro-optic transducer atthe low-temperature end, e.g., within the cryocooler or at anintermediate temperature, of which several types are well known in theprior art. These include several types of small semiconductor lasers(such as vertical cavity surface emitting lasers or VCSELs) andlight-emitting diodes (LEDs). (See FIG. 1B.) See, for example, Mukhanovet al., “Development of Energy Efficient Cryogenic Optical Data Link”,Proc. Superconductive Electronics Conference, 2013; Bunz et al., “LaserDiodes for Optical Readout of Cryoelectronics”, Proc. SPIE 2226,Infrared Readout Electronics II, 50 (Jun. 23, 1994).

Alternatively, one may send an optical fiber with a carrier optical wavedown to the cryogenic environment, and subject this carrier wave tomodulation in an electro-optic modulator device. Several types ofmodulator devices are known in the prior art, but most of these aresemiconductor devices that require an input voltage of order 1 V ormore, and may also dissipate significant levels of power. Again, onecould use a transistor amplifier to bridge the voltage gap, with thesame power limitations as above. (See FIG. 1C.) See U.S. Pat. Nos.4,810,978; 6,661,560; 5,210,637. Note that carrier mobility, may be lowat cryogenic temperatures, and thus semiconductor technology amplifiersmay have impaired performance.

See “Direct drive optical link for high-speed cryogenic data readout”,www.sbir.gov/sbirsearch/detail/377697, Navy ID N11A-022-0400; MarekOsinski and Gennady A. Smolyakov; “Injection locked microring lasers forultrafast modulation”, spie.org/x84982.xml.

The prior art does not provide an optical source or modulator that issensitive to an input signal of 1 mV or less, which dissipates verylittle heat and requires little or no input amplification to produce adetectable signal through an optical fiber of about 5-250 cm.

Very recently, electro-optic modulators based on the unique materialgraphene have been reported. Graphene comprises a single atomic layer ofa hexagonal lattice of carbon atoms (though multiple layers may beprovided in some cases), and can be viewed as a zero-gap semiconductorwith extremely high electron mobility. (In multiple layer structures,electron mobility is anisotropic). Graphene is also a strong opticalabsorber over a broad optical band, which is remarkable given its singleatomic layer. Together, these enable very small optoelectronic deviceswith low levels of power dissipation.

For example, U.S. Pat. No. 9,360,689, and U.S. patent application Ser.No. 2014/0056551 (Liu et al), “Graphene Based Optical Modulator”, (seeFIG. 2 ) provides a modulator which comprises either one or two graphenemonolayers embedded in an optical waveguide. See also,

U.S. Pat. No. 8,983,251 (Lu et al), “Electro-optical waveguide apparatusand methods thereof”;

U.S. Pat. No. 9,042,283 (Cho et al.), “Optical modulator includinggraphene”;

Jiaqi Wang, Zhenzhou Cheng, Zefeng Chen, Jian-Bin Xu, Hon Ki Tsang, andChester Shu, “Graphene photodetector integrated on silicon nitridewaveguide” J. Applied Physics 117, 144504 (2015);

Nathan Youngblood, Yoska Anugrah, Rui Ma, Steven J. Koester, and Mo Li,“Multifunctional Graphene Optical Modulator and Photodetector Integratedon Silicon Waveguides”, dx.doi.org/10.1021/n1500712u, Nano Lett. 2014,14, 2741-2746;

Steven J. Koester, Huan Li, and Mo Li, “Switching energy limits ofwaveguide-coupled graphene-on-graphene optical modulators”, OpticsExpress, Vol. 20, No. 18, 20330 (27 Aug. 2012);

Nicholas Hunter, Alexander S. Mayorov, Christopher D. Wood, ChristopherRussell, Lianhe Li, Edmund H. Linfield, A. Giles Davies, and John E.Cunningham, “On-Chip Picosecond Pulse Detection and Generation UsingGraphene Photoconductive Switches”, DOI: 10.1021/n1504116w, Nano Lett.2015, 15, 1591-1596;

Luo et al. Nanoscale Research Letters (2015) 10:199, DOI10.1186/s11671-015-0866-7; Muhammad Mohsin, Daniel Neumaier, DanielSchall, Martin Otto, Christopher Matheisen, Anna Lena, Giesecke, AbhayA. Sagade & Heinrich Kurz, “Experimental verification ofelectro-refractive phase modulation in graphene”, Scientific Reports 5,Article number: 10967 (2015), doi:10.1038/srep10967;

Liu, Ming, Xiaobo Yin, and Xiang Zhang. “Double-layer graphene opticalmodulator.” Nano letters 12.3 (2012): 1482-1485;

Liu, Ming, et al. “Graphene optical modulator.” SPIENanoScience+Engineering. International Society for Optics and Photonics,2011; Midrio, Michele, et al. “Graphene-assisted critically-coupledoptical ring modulator.” Optics express 20.21 (2012): 23144-23155;

Xu, Chao, et al. “Characteristics of electro-refractive modulating basedon Graphene-Oxide-Silicon waveguide.” Optics express 20.20 (2012):22398-22405;

Locatelli, Andrea, et al. “Graphene-assisted control of coupling betweenoptical waveguides.” Optics express 20.27 (2012): 28479-28484;

Gosciniak, Jacek, and Dawn TH Tan. “Theoretical investigation ofgraphene-based photonic modulators.” Scientific reports 3 (2013);

Midrio, Michele, et al. “Graphene-based optical phase modulation ofwaveguide transverse electric modes.” Photonics Research 2.3 (2014):A34-A40;

Gosciniak, Jacek, and Dawn TH Tan. “Graphene-based waveguide integrateddielectric-loaded plasmonic electro-absorption modulators.”Nanotechnology 24.18 (2013): 185202;

Mohsin, Muhammad, et al. “Graphene based low insertion losselectro-absorption modulator on SOI waveguide.” Optics express 22.12(2014): 15292-15297;

Ryzhii, Victor, et al. “Double-graphene-layer terahertz laser: concept,characteristics, and comparison.” Optics express 21.25 (2013):31567-31577;

Tamagnone, Michele, et al. “Fundamental limits and near-optimal designof graphene modulators and non-reciprocal devices.” Nature photonics 8.7(2014): 556-563;

Koester, Steven J., and Mo Li. “Waveguide-coupled grapheneoptoelectronics.” Selected Topics in Quantum Electronics, IEEE Journalof 20.1 (2014): 84-94;

Ye, Shengwei, et al. “Electro-absorption optical modulator usingdual-graphene-on-graphene configuration.” Optics express 22.21 (2014):26173-26180;

Cho, Seong-Ho, and Hyun-jong Chung. “Optical modulator includinggraphene.”

Chen, Pai-Yen, et al. “Graphene-based plasmonic platform forreconfigurable terahertz nanodevices.” ACS Photonics 1.8 (2014):647-654;

Kwon, Min-Suk. “Discussion of the epsilon-near-zero effect of graphenein a horizontal slot waveguide.” Photonics Journal, IEEE 6.3 (2014):1-9;

Phare et al, “Graphene Electro-Optic Modulator with 30 GHz Bandwidth”,Nature Photonics, vol. 9, pp. 511-514, August 2015 (See FIG. 3 ), showsa high-Q optical resonator, of which the graphene layer is a part.Applying a voltage tunes the Fermi energy of the graphene via the fieldeffect, which in turn affects the optical absorption in the graphene,and changes the Q and hence the impedance of the resonator, thusshifting the resonance slightly and modulating the transmitted opticalwave.

All of these disclose devices designed to operate at room temperature,with no cryogenic properties disclosed or predicted. Some of thesedevices are somewhat sensitive to small voltages, and further have otherimportant advantages, such as compact size and broad optical bandwidth.

There are several recent reports of graphene devices operating atcryogenic temperatures, including hybrid devices with superconductingmaterials. See, for example, see:

McKitterick et al., “Graphene Microbolometers with SuperconductingContacts for Terahertz Photon Detection”, published online July 2013 atarxiv.org/abs/1307.5012;

Calado et al., “Ballistic Josephson Junctions in Edge-ContactedGraphene”, published online June 2015 at arxiv.org/abs/1501.06817;

Chapman et al., “Superconductivity in Ca-Doped Graphene”, publishedonline August 2015 at arxiv.org/abs/1508.06931;

Weber et al., “Coupling graphene mechanical resonators tosuperconducting microwave cavities, published online April 2014 atarxiv.org/abs/1403.4792;

U.S. Pat. Nos. 5,210,637; 5,566,015; 5,568,302; 6,115,170; 6,285,487;6,515,788; 6,429,958; 4,906,930; 5,239,548; 5,247,183; 5,258,619;5,271,074; 5,477,158; 5,488,504; 5,491,334; 5,519,526; 5,519,529;5,710,426; 5,812,318; 5,917,195; 6,009,114; 6,168,081; 6,471,126;6,473,218; 6,563,622; 6,656,528; 6,661,560; 6,665,111; 6,710,367;6,819,460; 6,970,279; 6,977,767; 7,265,712; 7,368,280; 7,401,514;7,413,147; 7,420,994; 7,667,200; 7,709,799; 7,733,926; 7,781,736;7,781,739; 7,782,914; 7,820,970; 7,903,338; 7,920,616; 8,039,819;8,111,722; 8,116,003; 8,116,624; 8,182,917; 8,217,381; 8,223,342;8,237,324; 8,260,143; 8,260,144; 8,260,145; 8,303,514; 8,368,118;8,432,611; 8,471,238; 8,548,334; 8,554,022; 8,571,075; 8,633,573;8,705,983; 8,729,524; 8,755,662; 8,767,192; 8,780,677; 8,786,950;8,787,410; 8,792,525; 8,829,471; 8,867,026; 8,867,931; 8,873,131;8,919,428; 8,963,090; 8,963,265; 8,969,903; 9,020,362; 9,024,415;9,029,782; 9,042,683; 9,057,639; 9,083,457; 9,143,702; 9,182,541;9,188,953; 9,195,005; 9,195,052; 9,204,931; 9,212,948; 9,244,268;9,252,554; 9,252,561; 9,274,352; 9,291,836; 9,297,955; 9,303,309;9,318,872; 9,324,733; 9,395,473; 9,411,176; 9,426,545; 9,431,565;9,450,696; 9,477,101; 20020036814; 20020105948; 20020168134;20030042487; 20030094433; 20030186521; 20040095627; 20040208602;20050286603; 20060045807; 20060049981; 20060198397; 20070045474;20070242705; 20080157235; 20080298401; 20080310470; 20090067468;20090206262; 20090232191; 20090232507; 20090232510; 20090235721;20090283680; 20100074281; 20100171393; 20100200208; 20100231921;20100252750; 20100320443; 20110158653; 20110241072; 20120129269;20120133931; 20120138887; 20120140236; 20120194885; 20120205352;20120208027; 20120243874; 20120260983; 20120327608; 20120328301;20130002472; 20130003066; 20130004180; 20130042679; 20130050788;20130071083; 20130101247; 20130176836; 20130289671; 20130315597;20140007677; 20140013841; 20140022533; 20140023321; 20140060178;20140084163; 20140084325; 20140085693; 20140085865; 20140102191;20140103213; 20140105553; 20140199550; 20140202994; 20140218738;20140224989; 20140294338; 20140308682; 20140313468; 20140327862;20140341496; 20140374872; 20150063739; 20150093515; 20150125155;20150168747; 20150192532; 20150194789; 20150236791; 20150240086;20150303256; 20150318667; 20150382089; 20160005894; 20160033849;20160041095; 20160062009; 20160084761; 20160094342; 20160097963;20160099537; 20160103341; 20160115032; 20160116288; 20160118527;20160139487; 20160141835; 20160187256; 20160208383; 20160212512;20160226216; 20160245687; 20160259059; 20160298030; 20160307939;20160308006; and 20160308075.

Each reference cited herein is expressly incorporated herein byreference in its entirety.

SUMMARY OF THE INVENTION

The present technology provides a cryogenic optoelectronic data linkwhich supports transmission of data out of a cryogenic environment,e.g., less than 80K or less than 25K, or less than 12.5K, as a modulatedoptical signal. The optical modulator comprises a graphene sheet whoseoptical properties are modulated by a low voltage (e.g., <1 mV)electrical signal, operating on an optical signal that may be providedinternally to or externally from the cryocooler. The term “optical”includes wavelengths from 100 μm to 32 nm, but preferably within a rangeto which a silicon or germanium photodetector is responsive, e.g.,190-1700 nm. A preferred embodiment comprises, at the sending end, anoptical modulator based on at least one graphene sheet, configured tooperate at cryogenic temperatures less than about 25K, which issensitive to very small voltage levels of order 1 mV. The sensitivity isbased on the tunability of the Fermi energy of the graphene via thefield effect. A key aspect of the invention is the realization thatbecause of the unique electronic band structure of graphene (inparticular, the absence of an energy gap between the valence andconduction bands), the voltage sensitivity of a properly designedoptical modulator will be much greater at cryogenic temperatures than atroom temperature. Since the Fermi level smearing is approximately k_(B)T(and so is the noise energy of the device), a scaling analysis showsthat this modulator should become more sensitive, approximately as 1/T.Comparing a temperature of 4 K with room temperature (300 K), it isexpected that the sensitivity to improve by a factor of about 300/4=75.In contrast to a conventional semiconductor device, where the voltagescale ˜1V is set by the typical energy gap ˜1 eV, here the voltagesensitivity is ˜4 k_(B)T/e, to get above the thermal noise. This gives asensitivity ˜0.1 V at room temperature, ˜1 mV at 4K, and intermediatevalues in between.

Because of the improved sensitivity, in a preferred embodiment, thegraphene modulator may be used to directly interface with the outputfrom a superconducting circuit, such as a rapid single flux quantum(RSFQ) circuit based on niobium or niobium nitride trilayer Josephsonjunctions, without the need for any semiconductor transistor amplifier,as shown in the block diagram of FIG. 4 . However, the superconductingoutput circuit may preferably comprise one of several superconductingoutput amplifiers known in the art. These may include, for example, asingle flux quantum (SFQ)/DC converter, a superconducting quantuminterference device (SQUID) amplifier, a SQUID stack amplifier, asuperconducting-ferromagnetic transistor, or an nTron. See, McCaughan,Adam N., and Karl K. Berggren. “A superconducting-nanowirethree-terminal electrothermal device.” Nano Letters 14.10 (2014):5748-5753. These devices are low in power, and can output fast multi-GHzsignals of order 1 mV in amplitude. See, for example, U.S. Pat. Nos.5,936,458; 7,468,630; 7,724,083; 7,816,940; 8,971,977; 8,933,695; andWO2014/197047.

In a preferred embodiment of the invention, the modulator may comprise ahigh-Q optical resonator of which the graphene sheet is stronglycoupled, similar to that shown in FIG. 3 . Metallic electrodes maycomprise superconducting metals (such as niobium), so that the Q may behigher than that available at room temperature, and series resistanceshould be very small (and dominated by contact resistance). In this way,RC time constants should be very small, enabling modulation with a verybroad signal bandwidth, of as much as 100 GHz.

In another preferred embodiment, a modulator may be integrated with asuperconducting circuit on the same chip. For example, FIG. 5 shows anoptical waveguide on a chip comprising a superconducting circuit, whereone or more graphene-based modulators are coupled to the opticalwaveguide. If a plurality of modulators are resonant at slightlydifferent frequencies, and the optical carrier signal on the input tothe optical waveguide comprises a plurality of optical wavelengthscorresponding to the plurality of resonant frequencies, then a pluralityof modulated optical signals can be simultaneously transmitted on theoutput of the optical waveguide. This comprises a wavelength-dispersivemultiplexed (WDM) optical signal, which can be transmitted on an opticalfiber up to room temperature (see FIG. 6 ). Such WDM signals are knownin the prior art for optical fiber communication, and devices todemodulate such signals to recover the microwave-frequency communicationsignals are commercially available. See, for example, US patentapplications 2009/0103863; 2004/0114867; U.S. Pat. No. 6,778,318.Commercial WDM systems for up to 64 (or even 128) channels areavailable. A similar demodulation procedure may be applicable to theplurality of weak electronic signals from the plurality ofsuperconducting circuits. Such a multiplexed system, with only a singleoptical fiber for multi-carrier input and a single fiber for output, candramatically simplify the cryogenic packaging and system integration.

It is noted that an optical detector may also be created using a similarstructure or enhanced structure, e.g., an ultra-broadband photodetectordesign based on a graphene double-layer heterostructure. For example,see Liu, Chang-Hua; Chang, You-Chia; Norris, Theodore B.; Zhong,Zhaohui; “Ti-Graphene photodetectors with ultra-broadband and highresponsivity at room temperature”, Nat Nano, (2014/04), vol. 9, iss. 4,pp. 273-278, dx.doi.org/10.1038/nnano.2014.31, 10.1038/nnano.2014.31.www.nature.com/nnano/journal/v9/n4/abs/nnano.2014.31.html#supplementary-information.The detector is a phototransistor consisting of a pair of stackedgraphene monolayers (top layer, gate; bottom layer, channel) separatedby a thin tunnel barrier. Under optical illumination, photoexcited hotcarriers generated in the top layer tunnel into the bottom layer,leading to a charge build-up on the gate and a strong photogating effecton the channel conductance. The devices demonstrated room-temperaturephotodetection from the visible to the mid-infrared range, withmid-infrared responsivity higher than 1 A W⁻¹. See also,

Fengnian Xia, Thomas Mueller, Yu-ming Lin, Alberto Valdes-Garcia andPhaedon Avouris, “Ultrafast graphene photodetector”, NatureNanotechnology 4, 839-843 (2009);

Freitag, Marcus. et al. “Photoconductivity of biased graphene.” NaturePhotonics 7.1 (2013): 53-59;

Freitag, Marcus, Tony Low, and Phaedon Avouris. “Increased responsivityof suspended graphene photodetectors.” Nano letters 13.4 (2013):1644-1648;

Shiue, R., et al. “Integrated Graphene-Based Photonic Devices.”Photonics, Optoelectronics: 94;

Gulbahar, Burhan. “Nanoscale Optical Communications with Graphene.” V.National Communication Technologies Symposium, ITUSEM, Izmir, Turkey,May 2013;

Kalugin, Nikolai G., et al. “Graphene-based quantum hall effect infraredphotodetectors.” SPIE OPTO. International Society for Optics andPhotonics, 2012.

A quantum stark effect modulator or sensor may also be employed, using agrapheme layer heterostructure. Liu, Ming, et al. “A graphene-basedbroadband optical modulator.” Nature 474.7349 (2011): 64-67; Min,Hongki, et al. “Intrinsic and Rashba spin-orbit interactions in graphenesheets.” Physical Review B 74.16 (2006): 165310; Lu, Zhaolin, andWangshi Zhao. “Nanoscale electro-optic modulators based on graphene-slotwaveguides.” JOSA B 29.6 (2012): 1490-1496.

In a preferred embodiment of the invention, the sending module of thecryogenic optoelectronic data link is at a cryogenic temperature of lessthan 100 K, or even more preferred, less than 10 K. The receiving moduleof the cryogenic optoelectronic data link may be at room temperature, oralternatively could also be at a cryogenic temperature, or anytemperature less than room temperature. For example, one may consider adata link between two superconducting circuits on separate chips, butsharing the same cryogenic environment. Or the receiving module may belocated in a different cryogenic environment. In another example, thesending and receiving modules are located at substantially differenttemperatures, such as 4 K for the sending module and 30-100 K for thereceiving module, as might be the case if the optoelectronic data linksconnected a low-temperature superconducting circuit with ahigh-temperature superconducting circuit. In another example, theoptoelectronic data link might connect an ultra-low-temperature circuitoperating at 1 K or less (such as a sensor or quantum computing circuit)with a low temperature circuit operating at 4 K or above. These examplesare in no way exclusive, and just illustrate the wide range of potentialcryogenic applications for which the invention may solve the datainterface problem.

The optoelectronic demodulator circuit at the receiving module maycomprise a photoconductive (or photoresistive) device. Thephotoconductive device may also comprise a low-power optoelectronicdevice comprising at least one graphene sheet (see, for example, U.S.Pat. No. 8,053,782), which may also be operated at a temperature belowroom temperature. However, for a receiver at room temperature, the powerconstraint may be less critical, and other more conventional materialsmay suffice.

In general, the performance criteria are functionally specified, andwould generally involve the modulator, communication channel andreceiver, to set forth minimum performance criteria. In general, in adigital communication system, the raw bit error rate should be less than10⁻² or 10⁻³, and typically less than 10⁻⁶. This margin allows theperformance to be extended, at the expense of throughput, by use oferror detection and correction codes, as well as redundancy. Below 10⁻²bit error rate, it is likely that the required codes (encoding) forreliable communications would consume a sufficient amount of theavailable bandwidth to render the communication link infeasible.

The preferred modulators receive an input having a voltage of less than10 mV peak, and preferably less than about 1 mV peak, and are typicallynot amplified to a higher voltage. The lack of a bandgap in graphene andthe low noise at cryogenic temperatures mean that the properties,including optical properties, may be effectively modulated near 0V.Therefore, with such inputs, a suitable phase and/or amplitudemodulation may be achieved. For example, in a low noise environment, anamplitude change of 0.05 dB, 0.1 dB, or 0.2 dB should be detectable.Likewise, a phase shift of 0.05 radians, 0.1 radians, or 0.2 radiansshould be detectable.

In a preferred embodiment, the optical beam is frequency modulated (andat least during transitions, is phase modulated). Under suchcircumstances, the modulated signal is distinct from the carrier, andthe ratio or phase relationship is not directly relevant. Rather, thecarrier may be filtered from modulated signal, and the modulated signalis detectable if it is detectable or statistically distinguishable fromthe noise. The noise, in turn, derived from the three basic componentsof the system, the modulator, communication channel, and receiver. Sincethe modulator operates at cryogenic temperatures, which may be <11 K,there is a low thermal noise, and receiver noise may dominate theanalysis. Thus, the low voltage input signal can be effectively used,even if the fractional change in properties of the modulated material,which in this case is preferably a single atomic monolayer, and morepreferably graphene, is small.

In some cases, the system operates in an analog mode, or the performanceis not considered in terms of “bit” error rates, but rather analogdistortion. This may be true even if the modulating signal is generatedas digital pulses, e.g., from Josephson junctions. For example, in aradio receiver system, the pulses may represent a pulse modulatedoversampled representation of a radio frequency channel or band, and theoutput is to be demodulated as a radio frequency signal. This may beappropriate where the purpose of the superconducting electronics is todeconvolve multiple signals which are received together. In this case,the errors may be considered statistically instead of digitally, thougherror correction and detection codes would not be applicable to improveperformance, but perhaps redundant communication channels if subject touncorrelated errors might be.

It is therefore an object to provide an electro-optic communicationsystem, comprising: an optical modulation module, comprising at leastone layer of graphene, configured to modulate an optical carrier inputsignal based a voltage input signal, to generate a modulated opticalsignal; and an optical receiving module, configured to demodulate themodulated optical signal to produce a representation of the voltageinput signal. The optical receiving module may be adapted to beresponsive to changes in the amplitude of the voltage input signal ofless than about 10 mV, and more preferably less than about 1 mV, tocommunicate digital signals e.g., at a rate of at least 1 gigabits persecond at a bit error rate of less than 10⁻³. The receiver may employtraditional, room temperature technologies, while the modulator ispreferably maintained at a cryogenic temperature, preferably less than120K, more preferably less than 100K, and for example at <25K, <11K, andabout 4K. According to one embodiment, a superconducting contactmaterial induces at least a portion of the graphene into asuperconducting state.

It is also an object to provide an optical modulator, comprising: atleast one layer of graphene, modulated by a voltage input signal havingan amplitude less than about 10 mV, at a temperature of less than about100 K, and at a rate of at least 1 gigabits per second or a frequency ofat least 1 GHz, the at least one layer of graphene having an opticalproperty altered by the voltage input signal to produce a modulatedoptical signal from an optical carrier signal; and at least one opticalport configured to transmit the modulated optical signal.

It is a still further object to provide a cryogenic optoelectronicmodulator, comprising: at least one voltage input port, configured toreceive at least one input signal having a full scale magnitude lessthan about 10 mV; a pathway configured to carry the at least one inputsignal, which is superconducting at temperatures below 100K; an opticalcarrier input port, configured to receive an optical carrier wave; anelectro-optic modulator, configured to operate at a cryogenictemperature less than about 100 K, comprising at least one layer ofgraphene, configured to modulate the optical carrier wave in dependenceon at least the at least one input signal to produce a modulated opticalcarrier wave; and a modulated optical output port, configured totransmit the modulated optical carrier wave.

It is therefore an object to provide an electro-optic communicationmethod, comprising: providing an optical modulation module, comprisingat least one layer of graphene, configured to modulate an opticalcarrier input signal based a voltage input signal, to generate amodulated optical signal; modulating an optical carrier with the opticalmodulation module, and demodulating the modulated optical signal with anoptical receiving module, to produce a representation of the voltageinput signal.

It is also an object to provide an optical modulation method,comprising: providing at least one layer of graphene, having an opticalproperty modulated by a voltage input signal having an amplitude lessthan about 10 mV, at a temperature of less than about 100 K, and at arate of at least 1 gigabits per second or a frequency of at least 1 GHz,to produce a modulated optical signal from an optical carrier signal;and transmitting the modulated optical signal through at least oneoptical port.

It is a still further object to provide a cryogenic optoelectronicmodulation method, comprising: receiving at least one input signalhaving a full scale magnitude less than about 10 mV, communicated overat least a pathway which is superconducting at temperatures below 100K;receiving an optical carrier wave; and modulating the optical carrierwave with an electro-optic modulator, operating at a cryogenictemperature less than about 100 K, comprising at least one layer ofgraphene, dependent on at least the at least one input signal, toproduce a modulated optical carrier wave.

The at least one input signal may have a full scale magnitude less thanabout 1 mV.

The pathway configured to carry the at least one input signal, which issuperconducting at temperatures below 100K may be a high temperaturesuperconductor, a low temperature superconductor, and is preferablyniobium or niobium nitride.

The optical carrier input port may be a free space optic or a solidwaveguide, e.g., a fiber optic or conduit disposed on an integratedcircuit.

The optical carrier is typically generated outside of a cryogeniccryocooler, and fed in through a fiber optic. However, in some cases, acryogenic electromagnetic radiation source may be provided, and indeedin some cases the electromagnetic radiation source may comprisegraphene.

The electro-optic modulator may be configured to operate at a cryogenictemperature less than about 50K, 25K, 12K, or 4K (liquid helium), orbelow. Typically, microKelvin temperature operation is inefficient ifthere is incident electromagnetic radiation, though if carefullycontrolled, the optical heating may be less than the heat leakagethrough Ohmic contacts.

The electro-optic modulator may thus operate at a temperature near 4 K.

The modulated optical carrier may have a modulation of amplitude of atleast 0.2 dB. The modulated optical carrier may have a modulation ofphase of at least 0.02 radians. The modulated optical carrier may befrequency modulated, e.g., by a graphene ring oscillator.

The at least one input signal may have at least one frequency componentgreater than about 1 GHz. The modulated optical carrier wave mayrepresent the at least one frequency component greater than about 1 GHzwith a signal to noise ratio of at least 1 dB and a modulation ofamplitude of at least 0.2 dB or a modulation of phase of at least 0.02radians. The modulated signal may be a frequency modulated opticalsignal having an amplitude of greater than about −160 dB with respect tothe carrier optical signal in the modulated output, and preferablygreater than about −140 dB, and more preferably greater than about −120dB. The frequency modulated optical signal is preferably modulated at afrequency of at least 10 GHz, and more preferably at least 20 GHz, fromthe carrier optical frequency.

The at least one layer of graphene may be configured to change opticalproperties in response to the at least one input signal.

The at least one input signal may have a bandwidth of at least 10 GHz.

The at least one voltage input port may be connected to an output of asuperconducting circuit, without an intervening transistor amplifier.The superconducting circuit and the electro-optic modulator may beintegrated on a common substrate.

A plurality of voltage input ports may be provided, configured to carrya plurality of input signals that are multiplexed on the optical carrierwave by the electro-optic modulator. For example, the multiplexing maycomprise wavelength division multiplexing a plurality of differentoptical carrier wavelengths. In this case, a plurality of optical tingmodulators are provided, with different nominal modulation frequencies,e.g., 10 GHz, 20 GHz, 30 GHz, etc., and thus the respective inputsignals modulate the carrier optical signal in different bands,separable by filtering at the receiver.

According to another embodiment, a plurality of optical carriers areprovided at different wavelengths, and the modulated optical carrier mayhave a respective modulation for each of a plurality of optical carrierwavelengths.

According to further embodiment, the output modulated signal(s) arecarrier by a multimode optical fiber, and are separated according tomode at the receiver. In a multimode embodiment, each modulated signalcan have the same nominal modulation frequency (or be modulated atbaseband).

The system may comprise an integrated optical waveguide adjacent to atleast one layer of graphene.

An optical fiber may be provided which is configured to carry at leastone of the optical carrier wave and the modulated optical carrier wave.

The electro-optic modulator may comprise an optical resonator, e.g., amicro-ring resonator. The optical resonator may comprise a linearmicro-resonator. The electro-optic modulator may comprise a plurality ofoptical resonators, each driven by one of a plurality of input signals,where each resonator has a unique and different resonant frequency. Theplurality of resonant frequencies of the plurality of resonators maycomprise an optical frequency comb.

The at least one input signal may comprise a single-bit serial digitalsignal. Alternately, the at least one input signal may comprise a serialmulti-bit digital signal or a plurality of parallel digital signals. Theat least one input signal may also comprise one or more analog signals.

The energy per bit dissipated by the electro-optic modulator at thecryogenic temperature is preferably less than about 1 pJ/bit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block diagram of an electrical signal output from asuperconducting circuit in the prior art, using a cooled transistoramplifier.

FIG. 1B shows a block diagram of an optoelectronic signal output from asuperconducting circuit in the prior art, using a cryogenicsemiconductor laser and a cooled transistor amplifier.

FIG. 1C shows a block diagram of an optoelectronic signal output from asuperconducting circuit in the prior art, using a cryogenicelectro-optic modulator and a cooled transistor amplifier.

FIG. 2 shows a layout of a prior art graphene electro-optic modulator,from Liu et al (U.S. Pat. No. 9,360,689 B2, US 2014/0056551 A1).

FIG. 3 shows a conceptual diagram of the functioning of a prior artgraphene electro-optic modulator, from Phare et al (Nature Photonics,vol. 9, pp. 511-514, August 2015).

FIG. 4 shows a block diagram of one embodiment of an optoelectonicsignal output from a superconducting circuit according to the presentinvention, using a cryogenic graphene electro-optic modulator without acooled transistor amplifier.

FIG. 5 shows a conceptual diagram of a superconducting electronic chipwith integrated optical modulator readout.

FIG. 6 shows a conceptual diagram of a wavelength-division multiplexedoptical output line comprising a series of optical resonators withslightly different resonant frequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present technology may be used to provide an external interface fora superconducting circuit comprising an ultrafast microprocessor thatgenerates 64-bit digital words at a clock rate of 25 GHz, at atemperature of 4 K. Such a circuit could be designed usingRapid-Single-Flux-Quantum (RSFQ) logic, or one of its low-poweralternatives known as Reciprocal Quantum Logic (RQL), Energy-EfficientRSFQ, or Quantum Flux Parametron. See, for example, U.S. Pat. Nos.8,571,614; 7,843,209; 7,786,748.

Logic gates in these technologies exhibit switching energies of order10⁻¹⁸ J/gate, corresponding to signal levels of order 1 mV and 0.5 mAfor 2 ps. For a chip with one million gates at a clock rate of 25 GHz,this corresponds to a total power dissipation of 25 mW. The powerassociated with input/output lines and drivers at 4 K should becomparable to this. Taking a 64-bit data bus from 4 K to roomtemperature, 25 mW corresponds to 0.015 pJ/bit, an extremely smallvalue. In contrast, most conventional data links require energies muchgreater than 1 pJ/bit.

Table 1 presents estimates of the link energy budget for the CryogenicGraphene Modulator approach of the present invention, based on thesystem of FIG. 4 and the Micro-ring Modulator of FIG. 3 . This is for a25 Gb/s rate with 64 channels, modulated (using WDM) on the same opticaloutput line. Table 1 shows an estimated 1.66 pJ/b in total for theGraphene modulator approach, with only 0.31 pJ/b dissipated at 4 K. Thisis a crude estimate, but is quite promising. These numbers assume alaser source with a power of ˜10 mW, with a typical ˜20% energyefficiency.

TABLE 1 Data Link Energy Budget for Cryogenic Graphene Modulator withWDM Energy (pJ/b) Link Components @ 4K 0.31 Micro-ring Modulator 0.01Graphene Modulator Driver 0.3 Link Components at 300K 1.35 Laser Source(assuming 20% efficiency) 0.35 Photodetector/Receiver 1

In Table 2 below, the results of Table 1 are compared to otheralternative link technologies (FIGS. 1A, 1B, and 1C). Despite the crudeestimates in Table 2, the analysis makes clear that the integratedGraphene Modulator at 4 K is the most promising approach, in terms ofminimum energy, channel scaling, and compatibility of fabrication withthe superconducting circuits.

TABLE 2 Comparison of Alternative Approaches for Data Output Link SignalLevel Energy Channel Fabrication Approach (mV) (pJ/b) ScalingCompatibility Cryo- 1 mV 1.3 pJ/b, Single fiber Integrate Graphene butonly link for with super- Resonator 0.3 @ 4K. 64b word conducting @ 4Kusing WDM IC fab. (FIGS. 3-6) Electrical Progressive ~100 pJ/b, Amp.Chain + Separate Amplifier amplification distributed Wire/bit chips, @4K-300K up to 1 V in over Energy cost multi-stage (SiGe) multiplemultiple fromcable packaging (FIG. 1A) stages stages heat leaks Cryo-1-10 mV ~2-20 pJ/b VCSEL + fiber/ Separate VCSEL (Proposed estimatedbit + cable chips, @ 40-70K but not yet from 4-40K. complex fab (FIG.1B) verified) Cryo ~1 V ~1000 pJ/b Separate mod. Separate LiNbO₃Requires estimated for each bit. parts modulator amplification RequiresWDM (FIG. 1C) mod/demod for single fiber Cryo ~250 mV ~250 Separate mod.Separate polymer (est.) pJ/b est. for each bit. parts mod, not RequiresWDM yet verified mod/demod for (FIG. 1C) single fiber

FIG. 1A describes an electrical amplifier approach (without conversionto the optical domain), using cooled transistor amplifiers to achieve again of a factor ˜1000, or about 60 dB. SiGe heterojunction bipolartransistor (HBT) amplifiers tend to perform well at cryogenictemperatures, with performance superior to that at room temperature. Theamplifiers should preferably be distributed over two or more temperaturestages, in order to minimize heating the 4K stage. Most practicalcryogenic systems are built around commercial cryogenic refrigeratorsknown as cryocoolers, and most 4K cryocoolers have at least one coolingstage at an intermediate temperature, such as 60 K. However,low-attenuation broadband cables are unavoidably also good thermalconductors, making the tradeoffs unfavorable if minimal heat dissipationon the coldest stage is critical.

FIG. 1B describes a cryogenic laser which can be modulated by theelectrical signal from the superconducting circuit. However, mostsemiconductor laser drivers require voltages of ˜1V or more to generatesignificant modulation. One preferred type of semiconductor laser forintegrating with circuits is a vertical cavity surface emitting laser(VCSELs). Some VCSELs function at cryogenic temperatures, but theirpower dissipation is not really low enough for placement on a 4K stage,and would impose a relatively high load on the cryocooler. It has beenproposed to mount a VCSEL at an intermediate temperature ˜40-70 K,connected to the superconducting circuit by a low-attenuationsuperconducting transmission line. However, despite preliminary reportsof low voltage sensitivity for some VCSELs, it is likely that a coldtransistor preamplifier driver would still be required. Thisconfiguration requires a separate laser, fiber, input cable, andpreamplifier for each bit, making the packaging of a large number ofparallel bits rather problematic.

Two alternative electro-optic modulators (FIG. 1C) are described inTable 2: a classical EO material—LiNbO₃, and newer polymeric EOmaterials, such as amorphous polycarbonate and polymethacrylate (PMMA).See, e.g., U.S. Pat. No. 6,751,396; US 2002/0105948. The polymericmaterials are somewhat more voltage-sensitive than LiNbO₃. However,while the cryogenic performance of LiNbO₃ is well established, littlehas been demonstrated on the cryogenic electro-optic properties of thepolymeric materials. In any case, the necessary input voltage of theseestablished EO modulators is expected to be relatively large andessentially independent of temperature, so that an input preamplifierwould be required, as shown in FIG. 1C.

In contrast, the block diagram of FIG. 4 describes a cryogenic graphenemodulator without the need for an input preamplifier, which shouldrequire only ˜1 mV input amplitude at 4 K. The dissipation level at 4 Kis sufficiently low (˜0.3 mW or less) that the modulator can be closelyintegrated with the superconducting circuits without interfering withtheir temperature control or operation. A graphene modulator can bebased on one layer, or have multiple graphene layers, e.g., two layers.In either case, the mechanism is the same—field-effect shifting of theFermi energy of the graphene sheet. With one sheet, the voltage isapplied with respect to a metallic electrode across a thin dielectric.With two graphene sheets, the voltage is applied across the dielectricbetween the sheets, and the Fermi energy shifts in opposite directionsin the two sheets, as shown in FIG. 3 .

A single graphene sheet may be deposited by chemical vapor deposition(CVD) or plasma-enhanced chemical vapor deposition (PECVD). The graphenesheet may be deposited on top of a variety of substrates includingsilicon, silicon nitride, silicon dioxide, and aluminum oxide. See, forexample, US 2011/0303899; US 2014/0255621, Gao, Libo; Ni, Guang-Xin;Liu, Yanpeng; Liu, Bo; Castro Neto, Antonio H.; Loh, Kian Ping;“TI-Face-to-face transfer of wafer-scale graphene films”, Nature2014/01/09 505(7482) pp. 190-194, dx.doi.org/10.1038/nature12763. Adouble graphene sheet may be produced by depositing the first graphenesheet, followed by thin dielectric insulator such as aluminum oxide,followed by the second graphene sheet.

While direct vacuum deposition of one or more graphene sheets onto thedesired substrate is preferred, an optimized method for depositinghigh-quality graphene may not be available on some substrates.Therefore, an alternative method involves transfer of a graphene sheetdeposited on a different substrate, etched from that substrate, andtransferred to the desired substrate. See, e.g., U.S. Pat. Nos.8,906,245; 9,023,166; 8,926,852; and US 2014/0231002.

The optical signal may preferably be confined in an optical waveguide(see FIG. 2 ), which may comprise a core of silicon or silicon nitride,surrounded by silicon dioxide with a smaller index of refraction. Thegraphene layer(s) may border part of the core, so that modulatedabsorption of part of the light in the graphene layer may modulate theoptical signal.

It is further noted that the optical energy may be coupled to electronsin graphene to resemble Dirac fermion photon coupling, Antonio H. CastroNeto, “Graphene: Phonons behaving badly,” Nature Materials, vol. 6, p.176, March 2007; Gupta, Awnish, et al. “Raman scattering fromhigh-frequency phonons in supported n-graphene layer films.” Nanoletters 6.12 (2006): 2667-2673; Yan, Jun, et al. “Electric field effecttuning of electron-phonon coupling in graphene.” Physical review letters98.16 (2007): 166802; Sensale-Rodriguez, Berardi, et al. “Broadbandgraphene terahertz modulators enabled by intraband transitions.” Naturecommunications 3 (2012): 780; Berardi Sensale-Rodriguez, “Graphene-BasedOptoelectronics”, J. Lightwave Tech., 33(5):1100-1108 (Mar. 1, 2015);Andersen, David R. “Graphene-based long-wave infrared TM surface plasmonmodulator.” JOSA B 27.4 (2010): 818-823; Li, Wei, et al. “Ultrafastall-optical graphene modulator.” Nano letters 14.2 (2014): 955-959. Thishas implications for a graphene structure that carries the opticalenergy over some distance, and which can be modulated by externalinfluences over that distance. See also, Bart Ludbrook, Giorgio Levy,Pascal Nigge, Marta Zonno, Michael Schneider, David Dvorak, ChristianVeenstra, Sergey Zhdanovich, Douglas Wong, Pinder Dosanjh, CarolaStraBer, Alexander Stohr, Stiven Forti, Christian Ast, Ulrich Starke,Andrea Damascelli, “Evidence for superconductivity in Li-decoratedmonolayer graphene”, arXiv:1508.05925v2, PNAS, 112(38):11795-11799 (Sep.22, 2015); Jin-Soo Shin and Jin Tae Kim, “Broadband silicon opticalmodulator using a graphene-integrated hybrid plasmonic waveguide”,Nanotechnology 26 (2015) 365201 (8pp)doi:10.1088/0957-4484/26/36/365201.

In a preferred embodiment of the invention, portions of the opticalwaveguide may comprise a micro-ring resonator, similar to that shown inFIG. 3 . The resonant frequencies of such a resonator are determined bynλ=2πR, where R is the radius of the ring, λ is the wavelength of thelight in the resonator, and n is an integer. Note that alternativeresonator designs may produce similar results, such as a linearresonator of length L=nλ/2.

At least a portion of the resonator may be in contact with at least onegraphene sheet, the Fermi energy of which may be tuned using an appliedvoltage. This will change the optical absorption coefficient of thegraphene, modulating the resonant frequency and Q of the resonator.Furthermore, the resonator may be coupled to an optical transmissionline, also comprising an optical waveguide. If an applied microwavefrequency signal is applied to the graphene sheet(s), this will induce achange in the effective impedance of the resonator. If a resonantfrequency optical carrier signal is propagating along the transmissionline, such a change in the impedance will modulate the optical carriersignal.

In a further preferred embodiment of the invention, a plurality ofresonators may be coupled to the same optical transmission line, but theresonators are designed to have slightly different resonant frequencies(see FIG. 6 ). This may be achieved by fabricating circular resonatorswith slightly different radii (although the resonators do notnecessarily need to be circular). For example, a set of resonators maybe designed to have resonant frequencies that form a frequency comb, aset of frequencies with fixed frequency separations. An input opticalsignal can be generated that comprises such a frequency comb, as alsoshown in FIG. 6 . The frequency comb may establish a set of orthogonalfrequencies, and thus permit close spacing of the frequencies withouthigh risk of intersymbol interference. Such an optical frequency combgenerator is known in the prior art, see, e.g., US 2008/0285606. Onlythe component associated with a given resonator will couple to thatresonator, and produce a modulation that reflects the voltage applied tothe graphene sheet. So, an array of N resonators can simultaneouslycouple modulated signals to a single optical output line comprising themodulated frequency comb. This may be launched onto an optical fiber toa receiver at room temperature (or any other temperature). The receivercan demodulate each of the resonant frequencies, and reconstruct the Nsignals in parallel.

The example in FIG. 6 shows an optical wavelength of order 1550 nm,which corresponds to an infrared optical signal in the most common bandfor optical fiber communications. But the response of graphene is quitebroad across much of the infrared and visible spectrum, so that anyother optical bands could also be used.

There are significant advantages in the use of a single output fiber,particularly when the number N is large. For example, one may employ anarray of 64 resonators to output the elements of a 64-bit digital word.Sending them on a single line will help to assure that propersynchronization is maintained. Furthermore, device packaging andintegration may be greatly simplified.

FIG. 5 presents a preferred embodiment showing how the optical outputdevices may be integrated with superconducting circuits on the samechip, using standard wafer-scale lithography processes. Opticalwaveguide structures (the transmission line and the N resonators) mayfirst be fabricated by etching and deposition of materials such assilicon nitride and silicon dioxide, together with the graphene bilayers(see, e.g., WO 2014/089,454). The superconducting integrated circuits,which comprise a multilayer sequence of low-temperature deposition andpatterning processes, can then be processed on the same wafer, and dicedinto chips. Some examples of integrated superconducting depositionsequences based on niobium Josephson junctions are presented in thefollowing patents: U.S. Pat. Nos. 8,301,214; 7,615,385; and US2015/0119253.

There are a wide variety of cryogenic electronic systems that maybenefit from the use of low-voltage optical output, at temperatures from˜100 K down to 100 mK and below. The examples above focused on digitaloutputs from fast superconducting computers operating at 4K, based onniobium Josephson junctions. These optical output systems may also beused for digital outputs from precision cryogenic instruments such asfast digital samplers, digital radio receiver systems, and ultrafastswitching networks. Similar modulators can also be used for analogoutput, from superconducting and other cryogenic sensor arrays. Thesemay include, for example, SQUID sensors for biomagnetic imaging,single-photon sensors for astronomical imaging (based on superconductingnanowires, transition-edge sensors, kinetic inductance bolometers, orsuperconducting tunnel junctions), terahertz heterodyne receivers (basedon superconducting tunnel junctions, hot-electron microbolometers, orcooled Schottky diodes), and superconducting quantum interference filter(SQIF) arrays for sensitive radio receivers. See, e.g., U.S. Pat. Nos.6,665,553; 7,078,694; 7,991,013; 7,598,897; 7,362,125; 8,179,133;6,310,350. They may also include outputs from arrays of elements incryogenic quantum computing systems, which may operate at temperaturesof 100 mK or below. Advantageously, graphene may also be used toimplement qubits of the quantum computers, and therefore the graphenedeposition for external interfacing may occur in a common process withdeposition to form the qubit.

The range of systems that may benefit from the use of grapheneelectro-optic modulators may also include high-temperaturesuperconducting systems, based on cuprates that may operate as high as˜100 K, magnesium diboride devices that may operate up to 40 K, andiron-based superconductors that may operate up to ˜50 K, as well asnon-superconducting sensors that operate in the cryogenic regime up to˜100 K. See, e.g., CA 2600414; U.S. Pat. Nos. 8,155,318; 7,132,655.

These examples are not to be understood as limiting the invention, butrather as examples that illustrate the wide range of systems andapplications that may become clear to those skilled in the art.

Each reference cited herein is expressly incorporated herein in itsentirety, for its respective descriptions and teachings. The scope ofthe present invention includes the various combinations, subcombinationsand permutations of the elements disclosed herein either expressly orthrough incorporation. No element shall be deemed critical or requiredunless specified a being a necessary element.

What is claimed is:
 1. An optoelectronic modulator, comprising: anoptical port configured to communicate an optical signal; a signal portconfigured to communicate an electronic signal; at least one Josephsonjunction circuit configured to generate or receive the electronicsignal; a transducer comprising a hexagonal lattice of carbon atomshaving one or more layers and a zero band gap state, configured toassociate a state of optical rays of the optical signal with a state ofan electrical field on the hexagonal lattice of carbon atomscorresponding to the electronic signal, wherein a modulation of theoptical signal corresponds to a modulation of the electronic signal. 2.The optoelectronic modulator according to claim 1, wherein the opticalsignal is responsive to the electronic signal to produce at least one ofan amplitude change of at least 0.05 dB and a phase shift of at least0.05 radians in response to the modulation of the electronic signalhaving a maximum amplitude of less than 10 mV.
 3. The optoelectronicmodulator according to claim 1, wherein the optical signal and theelectronic signal are modulated with an information signal having abandwidth of at least 10 GHz.
 4. The optoelectronic modulator accordingto claim 1, wherein the optoelectronic modulator comprises a micro ringmodulator.
 5. The optoelectronic modulator according to claim 1, furthercomprising an optical waveguide adjacent to the sheet of the hexagonallattice of carbon atoms, configured to provide an optical interactionbetween the optical rays and the hexagonal lattice of carbon atoms. 6.The optoelectronic modulator according to claim 5, wherein the hexagonallattice of carbon atoms comprises a sheet of graphene deposited on asurface of the optical waveguide.
 7. The optoelectronic modulatoraccording to claim 6, wherein the optical waveguide comprises a fiberoptic communicating a wavelength division multiplexed signal.
 8. Theoptoelectronic modulator according to claim 1, further configured tocommunicate the electrical signal at the signal port with a cryogenicdevice at a temperature of less than 10 K.
 9. The optoelectronicmodulator according to claim 8, wherein the cryogenic device comprisesat least one of a qubit and a quantum computer.
 10. The optoelectronicmodulator according to claim 8, wherein the cryogenic device comprisesat least one of a superconducting digital circuit and a superconductingquantum interference detector (SQUID).
 11. The optoelectronic modulatoraccording to claim 8, wherein the cryogenic device comprises a cryogenicphotodetector.
 12. The optoelectronic modulator according to claim 8,wherein the modulator is integrated on a common substrate with thecryogenic device.
 13. The optoelectronic modulator according to claim 1,further comprising at least one single flux quantum circuit configuredto generate or receive the electronic signal.
 14. A method of modulatingoptical rays, comprising: receiving light to be modulated; receiving anelectronic modulation signal; providing modulator comprising a hexagonalatomic lattice having a field effect tunable Fermi energy k_(B)T,wherein k_(B) is the Boltzman constant and T is the temperature,dependent on the received electronic modulation signal supplying thefield; cooling the hexagonal atomic lattice below 100 K; interacting thehexagonal atomic lattice with the received light, in an opticalresonator, to thereby modulate the light according to the receivedelectronic modulation signal, to produce a modulated optical signal,having a sensitivity of ˜4 k_(B)T/e, wherein e is the elementary charge;and communicating the modulated optical signal through an optical port.15. The method according to claim 14, further comprising generating theelectronic modulation signal with a circuit comprising a Josephsonjunction.
 16. The method according to claim 14, further comprisinggenerating the electronic modulation signal with a single flux quantumcircuit comprising a Josephson junction.
 17. The method according toclaim 14, wherein the hexagonal atomic lattice comprises carbon and hasa zero band gap state.
 18. The method according to claim 14, wherein thehexagonal atomic lattice comprises graphene and is part of microringmodulator, and the light to be modulated is received through a waveguidehaving an optical surface, configured such that the grapheneoptoelectronically interacts with the light to be modulated through theoptical surface.
 19. An optoelectronic transducer, comprising: anelectrical interface circuit comprising at least one Josephson junction,configured to communicate an electronic modulation signal; an opticalresonator comprising a waveguide having a reflective surface configuredto guide light rays; and a hexagonal atomic lattice formed on thereflective surface of the waveguide, the hexagonal atomic lattice havinga Fermi energy k_(B)T, and a set of electrodes, wherein k_(B) is theBoltzman constant and T is the temperature, the hexagonal atomic latticeassociating a state of an electric field across the electrodes with astate of optical rays within the waveguide.
 20. The optoelectronictransducer according to claim 19, wherein the electrical interface isconfigured to receive the electronic modulation signal and modulate thelight rays.
 21. The optoelectronic transducer according to claim 19,wherein the waveguide is configured to receive the light rays andmodulate the electronic modulation signal.
 22. The optoelectronictransducer according to claim 19, wherein the electrical interfacecircuit is further configured to communicate the electrical modulationsignal with a cryogenic device at a temperature of less than 10 K.